Computer generated DOEs of the above kind are capable of performing complicated phase transformations of a radiation wave incident thereon such as a conversion of incident radiation wavefront having one shape into a wavefront of any other shape. DOEs of the specified kind are usually designed to have a high diffraction efficiency at a predetermined, most often, first diffraction order.
In order to obtain 100% diffraction efficiencies, DOEs suggested by Jordan et al and known as kinoforms have a periodic blazed surface relief with phase zones having a continuous profile ("Kinoform lenses", Appl. Opt., 9/8, August 1970, pp. 1883-1887). The depth of the phase zones in kinoforms is generally proportional to phase residues after modulo 2.pi. so that, in each phase zone, phase variations range is from 0 to 2.pi.. However, it is practically very difficult to produce high quality kinoforms with properly shaped continuous blazed profile.
It has, therefore, been suggested to quantize the ideal continuous phase profile of the DOEs into discrete phase levels as an approximation to the continuous profile. Manufacturing of such a multilevel DOE is based on a generation of a plurality of binary amplitude masks and their serial use for serial etching of a plurality of levels over the entire optical element. Thus, for example, a multilevel DOE disclosed in U.S. Pat. No. 4.895,790, is produced by means of M masks in M serial manufacturing cycles so that, at each manufacturing cycle, each previously produced level is divided into two levels with a smaller distance therebetween. Thereby, in each phase zone of the DOE, there are produced N=2.sup.M levels spaced by identical distances having and boundaries defining equiphase areas of the DOE. However, due to the fact that in such a multilevel DOE, all the phase zones have identical depth and number of phase levels, an amplitude of the diffracted wavefront cannot be changed independently of its phase and therefore, a desired distribution of overall diffraction efficiency of such a DOE cannot be achieved. Furthermore, when a multilevel DOE of the above kind has a varying grating period, such as for example in case of high numerical aperture diffractive lenses, maximal local diffraction efficiencies cannot be simultaneously obtained from all the phase zone, whereby overall diffraction efficiency of the DOE is reduced.
To provide for an independent control of an amplitude of diffracted wavefront, in a binary DOE, Brown, B. R. and Lohmann, A, W. have suggested a DOE in which the amplitude of the diffracted wavefront is controlled by an appropriate choice of the ratio between the widths of the levels (Brown, B. R. and Lohmann, A, W., "Complex spatial filtering of binary masks", Applied Optics, 5 June 1966, p. 967). However, with the number of phase levels being limited to two, the diffraction efficiency of the DOE cannot exceed 40.5%.
It is the object of the present invention to provide a new computer generated multilevel phase diffractive optical element, in which local diffraction efficiencies and consequently an overall diffraction efficiency can be arbitrarily controlled in the range from 0 to nearly 100% over the entire element.